Steel strip, sheet or blank for producing a hot formed part, part, and method for hot forming a blank into a part

ABSTRACT

A hot formed part produced from such a steel strip, sheet or blank, to the use of such a hot formed part, and to a method for forming such a steel blank or a preformed part made from such a blank, into a part.

The present invention relates to a steel strip, sheet or blank forproducing a hot formed part; a hot formed part; and a method forproducing a hot formed part.

There is an increasing demand for steel alloys that allow for weightreduction of automobile parts in order to reduce fuel consumption,whilst they provide at the same time improved protection of passengers.

In order to meet the automotive industry's requirements in terms ofimproved mechanical properties, such as improved tensile strength,energy absorption, workability, ductility and toughness, cold-formingand hot-forming processes have been developed to produce steels thatmeet these requirements.

In cold-forming processes, the steel is shaped into a product at nearroom temperature. Steel products produced in this way are for instancedual phase (DP) steels which have a ferritic-martensitic microstructure.Although these DP steels display a high ultimate tensile strength, theirbendability and yield strength are low which is undesirable since itreduces crash performance.

In hot-forming processes, steels are heated beyond theirrecrystallization temperature, and quenched to obtain desired materialproperties, usually by a martensitic transformation. The basics of thehot forming technique and steel compositions adapted to be used thereforwere already described in GB1490535.

A steel typically used for hot-forming is 22MnB5 steel. This boron steelcan be furnace-heated and is usually austenitized between 870-940° C.,transferred from furnace to forming tool, and stamped into the desiredpart geometry, while the part is at the same time cooled. The advantageof such boron steel parts produced this way is that they display a highultimate tensile strength for anti-intrusive crashworthiness due totheir fully martensitic microstructure, but at the same time theydisplay a low ductility and bendability which in turn results in alimited toughness and thus a poor impact-energy absorptivecrashworthiness.

Fracture toughness measurement is an useful tool to indicate the crashenergy absorption of steels. When the fracture toughness parameters arehigh, generally a good crash behavior is obtained.

In view of the above, it will be clear that there is a need for steelparts that display an excellent ultimate tensile strength, and at thesame time an excellent ductility and bendability, and in turn excellentcrash energy absorption.

It is therefore an object of the present invention to provide a steelstrip, sheet or blank that can be hot formed into a part that has acombination of an excellent ultimate tensile strength, ductility andbendability, thereby providing an excellent crash energy absorption whencompared to conventional cold-formed and hot-formed steels.

It is another subject of the present invention to provide a hot formedpart which is produced from such a steel strip, sheet or blank, and theuse of such a hot formed part as a structural part of a vehicle.

Yet another object of the present invention is to provide a method forhot-forming a steel blank into a part.

It has now been found that these objects can be established when use ismade of a low alloy steel that contains, in addition to carbon,manganese, chromium, titanium and nitrogen, relatively small amounts ofniobium and boron.

Accordingly, the present invention relates to a steel strip, sheet orblank for producing hot formed parts having the following composition inweight %:

C: 0.03-0.17,

Mn: 0.65-2.50,

Cr: 0.2-2.0,

Ti: 0.01-0.10,

Nb: 0.01-0.10,

B: 0.0005-0.005,

N: ≤0.01,

wherein Ti/N≥3.42,

and optionally one or more of the elements selected from:

Si: ≤0.1,

Mo: ≤0.1,

Al: ≤0.1,

Cu: ≤0.1,

P: ≤0.03,

S: ≤0.025,

O: ≤0.01,

V: ≤0.15,

Ni: ≤0.15

Ca: ≤0.05

the remainder being iron and unavoidable impurities.

The hot formed part produced from the steel strip, sheet or blank inaccordance with the present invention displays an improved combinationof tensile strength, ductility and bendability, and thereby impacttoughness when compared to conventional hot-formed boron steels.

Two automotive components for this steels are in mind, namely the frontlongitudinal bars and the B-pillar. For the front longitudinal,currently a cold-formed dual phase steel (DP800) is used and for theB-pillar a hot stamped 22MnB5 steel is used. The DP steel has a lowerenergy absorption, and using a higher strength steel (Ultimate TensileStrength >800 MPa) will enable more weight saving through downgaugingand enhanced passenger safety by higher crash energy absorption. On theother hand, for the B-pillar one currently used solution is using twotypes of steels—ultra high strength (˜1500 MPa) 22MnB5 for the upperpart and a lower strength (500 MPa) steel for the lower part. The twosteel blanks are joined by laser welding before hot stamping and thenthe hybrid blank is stamped into the B-pillar. By using this solution,during crash the upper part resists intrusion whereas the lower partabsorbs energy due to its high ductility. The current invention offersbetter performance and weight saving potential: the invented higherstrength steel can replace the lower strength steel of the lower partwith a higher energy absorption capability.

Preferably, the steel strip, sheet or blank for producing hot formedparts as described above has the following composition in weight %:

C: 0.05-0.17, preferably 0.07-0.15, and/or

Mn: 1.0-2.1, preferably 1.2-1.8, and/or

Cr: 0.5-1.7, preferably 0.8-1.5, and/or

Ti: 0.015-0.07, preferably 0.025-0.05, and/or

Nb: 0.02-0.08, preferably 0.03-0.07, and/or

B: 0.0005-0.004, preferably 0.001-0.003, and/or

N: 0.001-0.008, preferably 0.002-0.005

and optionally one or more of the elements selected from:

Si: ≤0.1, preferably ≤0.05,

Mo: ≤0.1, preferably ≤0.05,

Al: ≤0.1, preferably ≤0.05,

Cu: ≤0.1, preferably ≤0.05,

P: ≤0.03, preferably ≤0.015

S: ≤0.025, preferably ≤0.01

O: ≤0.01, preferably ≤0.005,

V: ≤0.15, preferably ≤0.05,

Ca: ≤0.01

the remainder being iron and unavoidable impurities.

Carbon is added for securing good mechanical properties. C is added inan amount of 0.03 wt % or more to achieve high strength and to increasethe hardenability of the steel. When too much carbon is added there isthe possibility that the toughness and weldability of the steel sheetwill deteriorate. The C amount used in accordance with the invention istherefore in the range of from 0.03-0.17 wt %, preferably in the rangeof from 0.05-0.17 wt %, and more preferably in the range of from0.07-0.15 wt %.

Manganese is used because it promotes hardenability and gives solidsolution strengthening. The Mn content is at least 0.65 wt % to provideadequate substitutional solid solution strengthening and adequate quenchhardenability, while minimising segregation of Mn during casting andwhile maintaining sufficiently low carbon equivalent for automotiveresistance spot-welding techniques. Further, Mn is an element that isuseful in lowering the Ac3 temperature. A higher Mn content isadvantageous in lowering the temperature necessary for hot pressforming. When the Mn content exceeds 2.5 wt %, the steel sheet maysuffer from poor weldability and poor hot and cold rollingcharacteristics that affect the steel processability. The Mn amount usedin accordance with the invention is in the range of from 0.65-2.5 wt %,preferably in the range of from 1.0-2.1 wt %, and more preferably in therange of from 1.2-1.8 wt %.

Chromium improves the hardenability of the steel and facilitatesavoiding the formation of ferrite and/or pearlite during pressquenching. In this respect it is observed that the presence of ferriteand/or pearlite in the microstructure is detrimental to mechanicalproperties for the targeted microstructure in this invention. The amountof Cr used in the invention is in the range of from 0.2-2.0 wt %,preferably in the range of from 0.5-1.7 wt %, more preferably in therange of 0.8-1.5 wt %.

Preferably, manganese and chromium are used in such an amount thatMn+Cr<2.7, preferably Mn+Cr is in the range of from 0.5-2.5, and morepreferably Mn+Cr is in the range of from 2.0-2.5.

Titanium is added to form TiN precipitates to scavenge out N at hightemperatures while the steel melt cools. Formation of TiN prohibitsformation of B₃N₄ at lower temperatures so that B, which is also anessential element for this invention, becomes more effective.Stoichiometrically, the ratio of Ti to N (Ti/N) addition shouldbe >3.42. In accordance with the invention the amount of titanium is inthe range of from 0.01-0.1 wt %, preferably in the range of from0.015-0.07 wt %, and more preferably in the range of from 0.025-0.05 wt%.

Niobium has the effect of forming strengthening precipitates andrefining microstructure. Nb increases the strength by means of grainrefinement and precipitation hardening. Grain refinement results in amore homogeneous microstructure improving the hot-forming behavior, inparticular when high localized strains are being introduced. A fine,homogeneous microstructure also improves the bending behavior. Theamount of Nb used in the invention is in the range of from 0.01-0.1 wt%, preferably in the range of from 0.02-0.08 wt %, and more preferablyin the range of from 0.03-0.07 wt %.

Boron is an important element for increasing the hardenability of steelsheets and for further increasing the effect of stably guaranteeingstrength after quenching. In accordance with the invention B is presentin an amount in the range of from 0.0005-0.005 wt %, preferably in therange of from 0.0005-0.004 wt %, more preferably in the range of from0.001-0.003 wt %.

Nitrogen has an effect similar to C. N is suitably combined withtitanium to form TiN precipitates. The amount of N according to theinvention is at most 0.01 wt %. Preferably the amount of N is in therange of 0.001-0.008 wt %. Suitably, N is present in an amount in therange of from 0.002-0.005 wt %.

In accordance with the present invention Mn, Cr and B are used in suchamounts that (B×1000)/(Mn+Cr) is in the range of from 0.185-2.5,preferably in the range of from 0.2-2.0, and more preferably in therange of from 0.5-1.5. The (B×1000)/(Mn+Cr) ratio as applied inaccordance with the present invention establishes an adequatehardenability of the steel.

The amounts of Si, Mo, Al, Cu, P, S, O, V, Ni and Ca, if present, shouldall be low.

Silicon is also added to promote hardenability and adequatesubstitutional solid solution strengthening. The Si amount used in theinvention is at most 0.1 wt %, preferably at most 0.5 wt %.

Aluminium is added to deoxidize the steel. The Al amount is at most 0.1wt %, preferably at most 0.05 wt %.

Molybdenum is added to improve the hardenability of the steel andfacilitate the formation of bainite. The Mo amount used in accordancewith the invention is at most 0.1 wt %, preferably at most 0.05 wt %.

Copper is added to improve hardenability and increase strength of thesteel. If present, Cu is used in accordance with the invention in anamount of at most 0.1 wt %, preferably at most 0.05 wt %.

P is known to widen the intercritical temperature range of a steel. P isalso an element useful for maintaining desired retained austenite.However, P may deteriorate the workability of the steel. In accordancewith the invention P should be present in an amount of at most 0.03 wt%, preferably at most 0.015 wt %.

The amount of sulphur needs to be minimised to reduce harmfulnon-metallic inclusions. S forms a sulfide based inclusions such as MnS,which initiates crack, and deteriorates processability. Therefore, it isdesirable to reduce the S amount as much as possible. In accordance withthe present invention the amount of S is at most 0.025 wt %, preferablyan amount of at most 0.01 wt %.

Steel products need to be deoxidised because oxygen reduces variousproperties such as tensile strength, ductility, toughness, and/orweldability. Hence, the presence of oxygen should be avoided. Inaccordance with the present invention, the amount of 0 is at most 0.01wt %, preferably at most 0.005 wt %.

Vanadium may be added to form V(C, N) precipitates to strengthen thesteel product. The amount of vanadium, if any, is at most 0.15 wt %,preferably at most 0.05 wt %.

Nickel may be added in an amount of at most 0.15 wt %. Ni can be addedto increase the strength and toughness of the steel.

Calcium may be present in an amount of up to 0.05 wt %, preferably up to0.01 wt %. Ca is added to spheroidize the sulphur containing inclusionsand to minimize the amount of elongated inclusions. However, thepresence of CaS inclusions will still lead to inhomogeneities in thematrix; it is thus best to reduce the amount of S.

According to a preferred embodiment, 1000*B divided by the sum of Mn andCr has to be between 0.185 and 2.5, preferably between 0.5 and 1.5. Thislimitation improves the hardenability of the steel.

Preferably, the steel strip, sheet or blank, is provided with a zincbased coating, an aluminium based coating or an organic based coating.Such coatings reduce oxidation and/or decarburization during a hotforming process.

It is preferred when the zinc based coating is a coating containing0.2-5.0 wt % Al, 0.2-5.0 wt % Mg, optionally at most 0.3 wt % of one ormore additional elements, the balance being zinc and unavoidableimpurities. The additional elements can be selected from the groupcomprising Pb or Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi. Pb, Sn,Bi and Sb are usually added to form spangles.

Preferably, the total amount of additional elements in the zinc alloy isat most 0.3 wt. %. These small amounts of an additional element do notalter the properties of the coating nor the bath to any significantextent for the usual applications.

When one or more additional elements are present in the zinc alloycoating, each is preferably present in an amount of at most 0.03 wt %,preferably each is present in an amount of at most 0.01 wt %. Additionalelements are usually only added to prevent dross forming in the bathwith molten zinc alloy for the hot dip galvanizing, or to form spanglesin the coating layer.

The hot formed part produced from a steel strip, sheet or blank inaccordance with the present invention has a microstructure comprising atmost 60% bainite, the remainder being martensite. Preferably, themicrostructure comprises at most 50 vol. % of bainite, the remainderbeing martensite. More preferably, the microstructure comprises at most40 vol. % of bainite, the remainder being martensite. The martensiteprovides a high strength, whereas the softer bainite improves theductility. The small strength difference between martensite and bainitehelps in maintaining a high bendability due to lack of weak phaseinterfaces.

The hot formed part in accordance with the present invention displaysexcellent mechanical properties. The part has a tensile strength (TS) ofat least 750 MPa, preferably of at least 800 MPa, more preferably of atleast 900 MPa, and further has a tensile strength of at most 1400 MPa.

The part suitably has a total elongation (TE) of at least 5%, preferably5.5%, more preferably at least 6% and most preferably at least 7%,and/or a bending angle (BA) at 1.0 mm thickness of at least 100°,preferably at least 115°, more preferably at least 130° and mostpreferably at least 140°.

It will be clear that the steel products in accordance with the presentinvention exhibit excellent crash energy absorption.

The present invention also relates to the use of hot formed parts asdescribed above, as structural part in the body-in-white of a vehicle.Such parts are made of the present steel strip, sheet or blank. Theseparts have a high strength, high ductility and a high bendability. Inparticular parts in the form of structural parts of vehicles are veryattractive since they exhibit excellent crash energy absorption and inturn, down-gauging and lightweighting opportunities based oncrashworthiness compared to the use of conventional hot-formed boronsteels and cold-formed multiphase steels.

The present invention also relates to a method for producing a part inaccordance with the present invention.

Accordingly, the present invention also relates to a method forhot-forming a steel blank or a preformed part into an part comprisingthe steps of:

-   -   a. heating the blank, or a preformed part produced from the        blank, as described above to a temperature T1 and holding the        heated blank at T1 during a time period t1, wherein T1 is higher        than the Ac3 temperature of the steel, and wherein t1 is at most        10 minutes;    -   b. transferring the heated blank or preformed part to a        hot-forming tool during a transport time t2 during which the        temperature of the heated blank or preformed part decreases from        temperature T1 to a temperature T2, wherein T2 is above Ar3 and        wherein the transport time t2 is at most 20 seconds;    -   c. hot forming the heated blank or preformed part into a heated        article; and    -   d. cooling the part in the hot-forming tool to a temperature        below the Mf temperature of the steel with a cooling rate (V3)        of at least 30° C./s.

In accordance with the present method it was found that through formingthe heated blank into a part as described above, complex shaped partswith enhanced mechanical properties can be obtained. In particular theparts exhibit excellent crash energy absorption and thus allow fordown-gauging and lightweighting opportunities based on crashworthinesscompared to the use of conventional hot-formed boron steels andcold-formed multiphase steel.

After the cooling of the part to a temperature below the Mf temperature,the part can for instance be further cooled to room temperature in air,or can be forcibly cooled to room temperature.

In the method according to the present invention, the blank to be heatedin step (a) is provided as an intermediate for the subsequent steps. Thesteel strip or sheet from which the blank is produced can be obtained bystandard casting processes. In a preferred embodiment the steel strip orsheet is cold-rolled. The steel strip or sheet can suitably be cut to asteel blank. A preformed steel part may also be used. The preformed partmay be partially or entirely formed into the desired geometry,preferably at ambient temperature.

The steel blank is heated in step (a) to a temperature T1 for a timeperiod t1. Preferably, in step (a) the temperature T1 is 50-100° C.higher than the Ac3 temperature of the steel, and/or the temperature T2is above the Ar3 temperature. When T1 is 50-100° C. above the Ac3temperature, the steel is fully or almost fully austenitized within thetime period t1, and the cooling during step (b) is easily possible. Whenthe microstructure is a homogenous austenitic microstructure theformability is enhanced.

Preferably, the time period t1 is at least 1 minute and at most 7minutes. Too long a time period t1 may result in coarse austeniticgrains, which will deteriorate the final mechanical properties

The heating apparatus to be used in step (a) may for instance be anelectric or gas powered furnace, electrical resistance heating device,infra-red induction heating device.

In step (b), the heated steel blank or preformed part is transferred toa hot-forming tool during a transport time t2 during which thetemperature of the heated steel blank or preformed part decreases fromtemperature T1 to a temperature T2, wherein the transport time t2 is atmost 20 seconds. Time t2 is the time needed to transport the heatedblank from the heating apparatus to the hot-forming tool (e.g. press)and till the hot-forming apparatus is closed. During the transfer of theblank or preformed part may cool from temperature T1 to temperature T2by the act of natural air-cooling and/or any other available coolingmethod. The heated blank or preformed part may be transferred from theheating apparatus to the forming tool by an automated robotic system orany other transfer method. Time t2 may also be chosen in combinationwith T1, t1 and T2 in order to control the microstructural evolution ofsteel at the commencement of forming and quenching. Suitably, t2 isequal or less than 12 seconds, preferably t2 is equal or less than 10 s,more preferably t2 is equal or less than 8 s, and most preferably equalor less than 6 s. In step (b), the blank or preformed part can be cooledfrom temperature T1 to a temperature at a cooling rate V2 of at least10° C./s. V2 is preferably in the range of from 10-15° C./s. When theblank or preformed part should be precooled, the cooling rate should behigher, for instance at least 20° C./s, up to 50° C./s or more.

In step (c) a heated blank or preformed part is formed into a parthaving the desired geometry. The formed part is preferably a structuralpart of a vehicle.

In step (d) the formed part in the hot-forming tool is cooled to atemperature below the Mf temperature of the steel with a cooling rate V3of at least 30° C./s. Preferably, the cooling rate V3 in step (d) is inthe range of from 30-150° C./s, more preferably in the range of from30-100° C./s.

The present invention provides an improved method of introducing duringhot-forming operation the desired bainitic phase into the steelmicrostructure. The present method enables the production of hot formedsteel parts displaying an excellent combination of high strength, highductility and high bendability.

One or more steps of the method according to the present invention maybe conducted in a controlled inert atmosphere of hydrogen, nitrogen,argon or any other inert gas in order to prevent oxidation and/ordecarburisation of said steel.

FIG. 1 shows a schematic representation of an embodiment of the methodaccording to the invention.

FIG. 2 shows a cross-section through a drop tower for axial crash tests.

In FIG. 1, the horizontal axis represents the time t, and the verticalaxis represents the temperature T. The time t and temperature T areindicated diagrammatically in FIG. 1. No values can be derived from FIG.1.

In FIG. 1, a steel blank or preformed part is (re)heated up to theaustenitizing temperature above Ac1 at a particular (re)heating rate.Once the Ac1 has been exceeded the (re)heating rate is lowered until theblank or preformed part has reached a temperature higher than the Ac3.Then the strip, sheet or blank is held at this particular temperaturefor a period of time. Subsequently, the heated blank is transferred fromthe furnace to the hot forming tool, during which cooling of the blankby air occurs to some extent. The blank or preformed part is thenhot-formed into a part and cooled down (or quenched) at a cooling rateof at least 30° C./s. After reaching a temperature below the Mftemperature of the steel, the hot-forming tool is opened and the formedarticle is cooled down to room temperature.

The different temperatures as used throughout the patent application areexplained below.

-   -   Ac1: Temperature at which, during heating, austenite starts to        form.    -   Ac3: Temperature at which, during heating, transformation of the        ferrite into austenite ends.    -   Ar3: The temperature at which transformation of austenite to        ferrite starts during cooling.    -   Ms: Temperature at which, during cooling, transformation of the        austenite into martensite starts.    -   Mf: Temperature at which, during cooling, transformation of the        austenite into martensite ends.

The invention will be elucidated by means of the following, non-limitingExamples.

EXAMPLES Steel Composition a (According to the Invention)

Steel blanks with dimensions of 220 mm×110 mm×1.5 mm were prepared froma cold-rolled steel sheet having the composition as shown in Table 1.These steel blanks were subjected to hot forming thermal cycles in a hotdip annealing simulator (HDAS) and an SMG press. The HDAS was used forslower cooling rates (30-80° C./s) whereas the SMG press was used forfastest cooling rate (200° C./s). The steel blanks were reheated to a T1of respectively 900° C. (36° C. above Ac3) and 940° C. (76° C. aboveAc3), soaked for 5 min. in nitrogen atmosphere to minimize surfacedegradation. The blanks were then subjected to transfer cooling for adrop in temperature of 120° C. in 10 s, so at a cooling rate V2 of about12° C./s and then subjected to cooling to 160° C. at the followingcooling rates V3: 30, 40, 50, 60, 80, 200° C./s. From the heat treatedsamples, longitudinal tensile specimens with 50 mm gauge length and 12.5mm width (A50 specimen geometry) were prepared and tested withquasistatic strain rate. Microstructures were characterized from theRD-ND planes. Bending specimens (40 mm×30 mm×1.5 mm) from parallel andtransverse to rolling directions were prepared from each of theconditions and tested till fracture by three-point bending test asdescribed in the VDA 238-100 standard. The samples with bending axisparallel to the rolling direction were identified as longitudinal (L)bending specimens whereas those with bending axis perpendicular to therolling direction were denoted as perpendicular (T) bending specimens.The measured bending angles at 1.5 mm thickness were also converted tothe angles for 1 mm thickness (=original bending angle×square root oforiginal thickness). For each type of test, three measurements were doneand the average values from three tests are presented for eachcondition.

For selected conditions (SMG press samples with reheating at 940° C.),J-integral fracture toughness and drop tower axial crash tests wereconducted. Compact tension specimens according to NFMT76J standard wereprepared from both longitudinal and transverse directions for fracturetoughness tests. For the transverse specimen, the crack runs along therolling direction and the loading is transverse to the rollingdirection, whereas the opposite applies for the longitudinal specimens.The specimens were tested according to ASTM E1820-09 standard at roomtemperature. The pre-cracks were introduced by fatigue loading. Thefinal tests were done with tensile loading with anti-buckle plates tokeep the stress in plane for sheet material. Three tests for eachconditions were done and following the guidelines in BS7910 standard theminimum values of three equivalents (MOTE values) for different fracturetoughness parameters are presented. A brief description of the fracturetoughness parameters is given below. CTOD is the Crack Tip OpeningDisplacement and is a measure of how much the crack opens at eitherfailure (if brittle) or maximum load. J is the J-integral and is ameasure of toughness that takes account of the energy, so it iscalculated from the area under the curve up to failure or maximum load.KJ is the stress intensity factor determined from the J integral usingan established expression, given as KJ=[J(E/(1−v²))]^(0.5) where E isthe Young's modulus (=207 GPa) and v is the Poisson's ratio (=0.03).K_(q) is the value of stress intensity factor measured at load P_(q),where P_(q) is determined by taking the elastic slope of the loadingline, then taking a line with 5% less slope and defining P_(q) as theload where this straight line intersects the loading line.

Drop tower axial crash tests were done in SMG-pressed condition with aload of 200 kg and a loading speed of 50 km/hour for the load to hit thecrash boxes having a closed top hat geometry (FIG. 2) with 500 mm height(transverse to the rolling direction). The dimensions of thecross-section of the drop tower are given in FIG. 2 in millimetres(t=1.5 mm, R₀=3 mm). The back plates of 100 mm width were spot-welded tothe profiles to prepare the crash boxes.

For some selected conditions, a paint bake thermal cycle was also givento the samples, and the tests were done as will be reflected from theresults directly.

Steel Compositions B and C (not According to the Invention)

For comparison reasons a commercially available cold-formedCR590Y980T-DP (steel composition B and commonly known as DP1000 steel)was also tested since it has a similar strength level as the steel blankin accordance with the invention. In addition, and also for comparativereasons, a standard hot-formed 22MnB5 steel product (steel compositionC) was tested.

In Table 1, the chemical compositions in wt % of steel compositions A-Care specified.

In Table 2, the transformation temperatures of steel composition A areshown.

The results of the various tests are presented in Tables 3 to 8.

In Table 3, the yield strength (YS), ultimate tensile strength (UTS),uniform elongation (UE), and total elongation (TE) are shown for steelcomposition A after a variety of cooling rates V3. In addition, Table 3shows the microstructure in terms of martensite (M) and bainite (B). Itwill be clear from Table 3 that an ultimate tensile strength of greaterthan 800 MPa was achieved at the different cooling rates V3.

In Table 4, bending angles (BA) at 1.0 mm thickness are shown for steelcomposition A as obtained after different cooling rates V3. It is clearfrom Table 4 that high bending angles of greater than at least 130° wereachieved for both the longitudinal (L) and transverse (T) orientations.

In Table 5, the various mechanical properties have been shown for steelcomposition A after said composition has been subjected to a horformingand baking treatment simulating the paint baking treatment used duringautomobile manufacturing. Steel composition A was heated to 900° C.,soaked for 5 min. and then cooled at a V3 of 200° C./s, following thetransfer cooling. The baking treatment was carried out at 180° C. for 20minutes. From Table 5, it will be clear that approximately the sameminimum levels of yield strength YS), ultimate tensile strength (UTS),ultimate elongation (UE), total elongation (TE) and bending angels (BA)are also achieved after steel composition A has been subjected to abaking treatment. This means that in automotive manufacturing afterpaint baking, the properties claimed will be ensured in servicecondition.

In Table 6, the various mechanical properties of steel compositions B(DP1000) and C (22MnB5) are shown. These steel compositions B and C weretested under the same test conditions as steel composition A. When thecontents of Tables 4 and 6 are compared it will become immediatelyevident that the steel part in accordance with the present invention(steel composition A) constitutes a major improvement in terms ofbendability when compared with conventional cold-formed steel productsDP1000 (steel composition B) and conventional hot-formed steel product22MnB5 (steel composition C).

From Table 7, it is also clear that the fracture toughness parameters ofthe steel part in accordance with the present invention (steelcomposition A) is also higher than that of blanks made of DP1000 (steelcomposition B).

In Table 8, the crash behavior of the steel compositions A and B isshown. From Table 8 it is clear that the crash behavior of steelcomposition A is better than that of DP1000 (steel composition B) inboth hot pressed as well as hot pressed and baked conditions. The bakingconditions are the same as described here above. The crash boxes ofsteel composition A did not show any indication of cracking after thetests, whereas the crash boxes of DP1000 (steel composition B) showedsevere cracking in the folds. Moreover, steel composition A shows ahigher energy absorption capability.

The high and improved crash behavior of hot formed steel composition Ain accordance with the present invention when compared to theconventional steel products of similar strength is due to the higherbending angle and higher fracture toughness properties. In this respectit is observed that during a crash, the steel component need to foldwhich is determined by its bendability, whereas on the other hand theenergy absorption capability before failure is determined by itsfracture toughness parameters.

In view of the above, it will be clear to the skilled person that thesteel products in accordance with the present invention constitute aconsiderable improvement over conventionally known cold-formed andhot-formed steel products.

TABLE 1 chemistry (wt %) Steel C Mn Si Nb B Cr Ti N Remainder A 0.0751.48 — 0.05 0.0025 1.01 0.03 0.0045 Fe + impurities B 0.15 2.3 0.1 0.01— — 0.015 0.0035 Fe + impurities C 0.23 1.25 0.2 — 0.003 — — 0.004 Fe +impurities

TABLE 2 Transformation temperatures steel composition A A_(c1) (° C.)A_(c3) (° C.) M_(s) (° C.) M_(f) (° C.) 770 864 486 287

TABLE 3 Mechanical properties and microstructures for steel compositionA T1 V3 YS UTS UE TE Microstructure (° C.) (° C./s) (MPa) (MPa) (%) (%)(vol. %) 900 30 696 893 2.8 5.6 55M + 45B 900 40 699 911 2.8 5.6 73M +27B 900 50 741 955 3.2 6.2 79M + 21B 900 60 772 998 3.2 5.8 91M + 9B 900 80 784 1003 3.7 6.4 94M + 6B  900 200 879 1090 3.2 6.1 100M 940 30757 962 3.6 6.9 60M + 40B 940 40 763 975 3.7 6.8 70M + 30B 940 50 741985 4.4 8.1 82M + 18B 940 60 782 1006 4.4 8.3 93M + 7B  940 80 777 10214.4 8.1 960M + 4B  940 200 892 1089 3.2 6.3 100M

TABLE 4 Bending angles for steel composition A BA BA BA BA (1.5 mm) (1.5mm) (1 mm) (1 mm) T1 V3 L sample T sample L sample T sample (° C.) (°C./s) (°) (°) (°) (°) 900 30 126.8 123 155.3 150.7 900 40 123.5 123.5151.2 151.2 900 50 126.2 126.4 154.5 154.8 900 60 123 124.1 150.7 152900 80 119.2 115.3 146 141.3 900 200 111.7 113 136.8 138.5 940 30 120.7122.4 147.8 149.9 940 40 127.8 121 156.5 148.1 940 50 121.2 125.9 148.5154.2 940 60 122.6 120.5 150.2 147.6 940 80 118.6 132.5 145.3 162.3 940200 122.1 117.9 149.5 144.4

TABLE 5 Mechanical properties Steel composition A after baking BA BA BABA (1.5 mm) (1.5 mm) (1 mm) (1 mm) YS UTS UE TE L sample T sample Lsample T sample (MPa) (MPa) (%) (%) (°) (°) (°) (°) 937 1072 2.5 5.7113.6 116 139.1 142.1

TABLE 6 Mechanical properties Steel compositions B (DP1000), and C(22MnB5) BA BA BA BA (1.5 mm) (1.5 mm) (1 mm) (1 mm) YS UTS UE TE Lsample T sample L sample T sample Steel (MPa) (MPa) (%) (%) (°) (°) (°)(°) DP1000 747 1022 7.3 14.4 65.4 74.4 71.1 81.5 22MnB5 912 1374 4.1 6.683.5 69.2 102.3 84.7

TABLE 7 Fracture toughness parameters for steel compositions A and B(DP1000) Orien- CTOD J KJ K_(Q) Steel tation (mm) (J/mm²) (MPa ·m^(0.5)) (MPa · m^(0.5)) Compo- L 0.361 0.638 381 90 sition A Compo- T0.245 0.434 314 104.7 sition A DP1000 L 0.139 0.231 229 86 DP1000 T0.146 0.243 235 79.2

TABLE 8 Crash test results for steel compositions A and B (DP1000) Meanforce at Visual Steel Condition 1.5 mm (NM) observation Steel heated and107 good folding; compo- pressed no cracking sition A Steel heated, 98good folding; compo- pressed no cracking sition A and baked DP1000 asannealed 82 severe cracking in folds

1. The steel strip, sheet or blank for producing hot formed parts havingthe following composition in weight %: C: 0.03-0.17, Mn: 0.65-2.50, Cr:0.2-2.0, Ti: 0.01-0.10, Nb: 0.01-0.10, B: 0.0005-0.005, N: ≤0.01,wherein Ti/N≥3.42, and optionally one or more of the elements selectedfrom: Si: ≤0.1, Mo: ≤0.1, Al: ≤0.1, Cu: ≤0.1, P: ≤0.03, S: ≤0.025, O:≤0.01, V: ≤0.15, Ni: ≤0.15 Ca: ≤0.15 the remainder being iron andunavoidable impurities.
 2. The steel strip, sheet or blank according toclaim 1, wherein: C: 0.05-0.17, and/or Mn: 1.00-2.10, and/or Cr:0.5-1.7, and/or Ti: 0.015-0.07, and/or Nb: 0.02-0.08, and/or B:0.0005-0.004, and/or N: 0.001-0.008, Ca: ≤0.01.
 3. The steel strip,sheet or blank according to claim 1, wherein the sum of the amount of Mnand Cr is less than 2.7.
 4. The steel strip, sheet or blank according toclaim 1, wherein Mn, Cr and B are used in such amounts that(B×1000)/(Mn+Cr) is in the range of from 0.185-2.5.
 5. The steel strip,sheet or blank according to claim 1, provided with a zinc based coatingor an aluminium based coating or an organic based coating.
 6. The steelstrip, sheet or blank according to claim 5, wherein the zinc basedcoating is a coating containing 0.2-5.0 wt % Al, 0.2-5.0 wt % Mg,optionally at most 0.3 wt % of one or more additional elements, thebalance being zinc and unavoidable impurities.
 7. A hot formed partproduced from a steel strip, sheet or blank according to claim 1, thehot formed part having a tensile strength of at least 750 MPa.
 8. Thehot formed part according to claim 7, having a total elongation (TE) ofat least 5% and/or a bending angle (BA) at 1.0 mm thickness of at least100°.
 9. The hot formed part according to claim 7, the hot formed parthaving a microstructure comprising at most 60% bainite, the remainderbeing martensite.
 10. A method of use of a hot formed part according toclaim 7, comprising forming the hot formed part into a structural partin a body-in-white of a vehicle.
 11. A method for hot-forming a steelblank or a pre-formed part into a part comprising the steps of: a.heating the blank, or a pre-formed part produced from the blank, whereinthe blank or the blank from which the pre-formed part is produced isaccording to claim 1, to a temperature T1 and holding the heated blankat T1 during a time period t1, wherein T1 is higher than the Ac3temperature of the steel, and wherein t1 is at most 10 minutes; b.transferring the heated blank or pre-formed part to a hot-forming toolduring a transport time t2 during which the temperature of the heatedblank or preformed part decreases from temperature T1 to a temperatureT2, wherein the transport time t2 is at most 20 seconds; c. hot formingthe heated blank or preformed part into a part; and d. cooling the partin the hot-forming tool to a temperature below the Mf temperature of thesteel with a cooling rate of at least 30° C./s.
 12. The method accordingto claim 11, wherein the temperature T1 in step (a) is 50-100° C. higherthan the Ac3 and/or the temperature T2 is above Ar3.
 13. The methodaccording to claim 11, wherein the time period t1 in step (a) is atleast 1 minute and at most 7 minutes and/or the time period t2 in step(b) is at most 12 seconds.
 14. The method according to claim 11, whereinthe part is cooled in step (d) with a cooling rate in the range of30-150° C./s.
 15. A vehicle comprising at least one hot formed partaccording to claim
 7. 16. A vehicle comprising at least one hot formedpart produced according to claim
 11. 17. The steel strip, sheet or blankaccording to claim 1, wherein: C: 0.07-0.15, Mn: 1.20-1.80, Cr: 0.8-1.5,Ti: 0.025-0.05, Nb: 0.03-0.07, B: 0.001-0.003, N: 0.002-0.005, and Ca:≤0.01.
 18. The steel strip, sheet or blank according to claim 1, whereinthe sum of the amount of Mn and Cr is between 0.5 and 2.5.
 19. The steelstrip, sheet or blank according to claim 1, wherein Mn, Cr and B areused in such amounts that (B×1000)/(Mn+Cr) is in the range of from0.5-1.5.
 20. The hot formed part produced from a steel strip, sheet orblank according to claim 1, the part having a tensile strength of atmost 1400 MPa.
 21. The hot formed part according to claim 7 having atotal elongation (TE) of at least 7% and/or a bending angle (BA) at 1.0mm thickness of at least 140°.
 22. The hot formed part according toclaim 7, the part having a microstructure comprising at most 40%bainite, the remainder being martensite.
 23. The method according toclaim 11, wherein the time period t1 in step (a) is at least 1 minuteand at most 7 minutes and/or the time period t2 is between 2 and 10seconds.
 24. The method according to claim 11, wherein the part iscooled in step (d) with a cooling rate of 30-100° C./s.